Geological Survey Professional Paper 160Geologic History of the Yosemite Valley

EARLY HISTORY OF THE YOSEMITE VALLEY

ANTIQUITY OF THE SIERRA NEVADA AND THE YOSEMITE VALLEY

The story of the evolution of the Yosemite Valley is
so intimately interwoven with the history of the rise of the Sierra
Nevada that it can scarcely be told with out reference to that larger
history. Indeed, every stage in the development of the valley
corresponds to a definite epoch in the growth of the range and assumes
its real significance only when considered in the light of that
relationship. It is but logical, therefore, to begin with a brief sketch
of the history of the Sierra Nevada.

At once arises the question of the length, of time
involved. How far back does this history take us? Mountains are by many
people supposed to stand forever, permanent and unchanging. "As old as
the eternal hills" is a familiar phrase. Yet to those who study the
earth nothing is more evident than that mountains have limited,
measurable spans of life. They come into being by upheaval and, through
the erosive action of streams, glaciers, and atmospheric agencies, are
worn down by degrees, sometimes to mere hills or even to plains. The
Sierra Nevada is decidedly a new range in the geologic sense, a still
youthful feature of the continent. Its youth is manifest from the fact
that it has not yet lost significantly in height by erosion. Though
deeply furrowed by canyons, it still ranks with the highest ranges in
this country and bears summits that retain in part the subdued outlines
which they had acquired prior to their elevation. The Sierra Nevada has
stood with its present height and form about a million yearsan
astounding stretch of time compared with the few thousand years of
recorded human history, yet very brief compared with the total length of
geologic history, which runs into billions of years. Viewed in its
proper historic perspective the Sierra Nevada is but the latest of
several successive mountain ranges, or mountain systems, that have in
turn occupied the same place. Each of these ancestral mountain systems
must have been in existence a very long time, for each was reduced to
ridges and hills of only moderate height, in part to an undulating
lowland. The time required for their demolition was doubtless between
50,000,000 and 100,000,000 years for each system.

The major facts as to the character, structure, and
history of at least two of these ancestral mountain systems are
indicated by certain rock formations that are the roots, so to speak, of
those earlier mountains, incorporated in the present range. And the
approximate time of creation of each of these earlier mountain systems
is indicated by the fossil remains that are preserved in their rocks,
each geologic epoch being known by its characteristic life forms.

In the table of geologic time divisions on page 23
the outstanding events in the geologic history of the
Sierra region are set forth in chronologic order,
each referred to its proper era, period, and epoch as definitely as the
knowledge at hand permits. The figures for the duration of the
successive time divisions are taken from the table which the late
Professor Barrell30 compiled from calculations of the age of uranium
minerals from different parts of the world. The age of these minerals is
computed from the ratio of lead to uranium present in them, the rate at
which uranium breaks down and is reduced to lead by atomic
disintegration being accurately known. The figures given are really the
means of the maxima and minima of Professor Barrell's table, these means
agreeing closely with the results obtained from the more refined
calculations made by Holmes and Lawson.31 These measures of
geologic time are much greater than those that have been current among
scientists in the past, but they doubtless afford much closer
approximations to the truth than the shorter measures, for they are of
an order of magnitude that is consistent with many geologic facts,
notably with the extremely slow rate at which mountains are worn
down.

It will be seen from the table that the first of the
two ancestral mountain systems of which the roots are still recognizable
came into being in the Permian epoch, near the end of the Paleozoic era,
more than 200,000,000 years ago. It was formed by the uplifting and
folding of a great series of layers of slate, shale, and
sandstoneoriginally mud, silt, and sand derived from a land mass lying
mostly to the west of the present border of the continent and laid down
in an arm of the Pacific Ocean. Folded in with these sediments, which
aggregated thousands of feet in thickness, were beds of lime (calcium
carbonate) deposited presumably in shallow water at times when the land
was low and the streams brought down but little sand or mud.

In the long stretches of time that ensued the
wrinkles in the earth's crust thus produced were in large part worn away,
and finally the region again sank below the level of the sea and became
a place of deposition. For millions of years new layers of mud, silt,
and sand, together with beds of volcanic material, accumulated upon the
submerged remnants of the first mountain system, and then, at the end of
the Jurassic period, about 130,000,000 years ago, there came another
upheaval, the new strata were folded and crumpled and were invaded by
molten granite from below, and there arose a second system of mountain
ranges that occupied most of eastern California and large areas in
adjoining States. Throughout the Cretaceous period, which followed upon
the Jurassic, this second mountain system was being gradually worn down,
until by the beginning of the Tertiary period only ridges of moderate
height were left.

The region is tilted to the west and assumes mountainous height at
its eastern margin.

Volcanic eruptions begin anew, and the northern half of the region
is covered by successive flows of andesitic lava and mud.

Prolonged interval marked by minor warpings of the earth's
crust, up and down. The land is subject to continued erosion and the
rhyolitic materials are mostly worn away.

The region, together with the country to the east of it, is slowly
upwarped to moderate heights. Volcanoes burst forth in the northern
part and cover the land repeatedly with rhyolitic lava, mud, and ash.

The mountain ranges are worn down gradually and the region, as a
whole is reduced to a lowland. The bulk of the sedimentary rock,
several thousand feet in thickness, is carried away by the streams,
and the granite is uncovered over large areas.
The new sediments, together with remnants of the old, are folded and,
crumpled into parallel, northwestward-trending mountain ranges.
Molten granite invades the folds from below.

More sediments are laid down as the sea bottom progressively sinks.

1,000,000

Pliocene.

Tertiary.

7,000,000

Miocene.

12,000,000

Oligocene.

16,000,000.

Eocene.

23,000,000.

Mesozoic

Cretaceous.

75,000,000

Jurassic.

40,000,000

Triassic.

The mountains are slowly worn down to hills. The land finally
sinks below the sea and new sediments are deposited.

40,000,000

Paleozoic

Carboniferous.

Permian.

The sediments are uplifted arid folded into the form of mountain ranges.

415,000,000

Pennsylvanian.Mississippian.

Sediments, mainly outwash from the continent, accumulate to
thicknesses of thousands of feet on the floor of the Pacific Ocean.

Devonian.

Silurian.

Ordovician.

Cambrian.

Proterozoic

Algonkian.

Nothing definite known.

Archean.

The present Sierra Nevada was not formed until a long
time thereafter. It assumed its present height and form about the dawn
of the Quaternary period; but throughout most of the Tertiary period,
especially in the later half, the region was the scene of repeated
disturbances and minor mountain-building movements that finally led up
to the culminating uplift. It is with these preliminary happenings in
the Tertiary period, which may be traced back with some confidence fully
20,000,000 years, that the history of the Sierra
Nevada properly begins, for it was in consequence of
those happenings that many features which are now
part and parcel of the physiognomy of the range were
developedamong others the Yosemite upland, the parallel crests of
the High Sierra, and the course of the Merced River, including that part
in which the Yosemite Valley was finally elaborated.

It will be helpful to the reader pursuing this story
to have at the outset some insight into the structure and general
make-up of the Sierra Nevada, and accordingly these will
first be explained.